Preparation method and application of biomass carbon composite sodium ion battery positive electrode material

Abstract

The application relates to a preparation method and application of a biomass carbon composite sodium ion battery positive electrode material, and relates to a preparation method and application of a biomass carbon composite sodium battery positive electrode material, and aims to solve the technical problems of a complex carbon composite modification process and high cost of an existing phosphate sodium ion battery positive electrode material. The method comprises the following steps: high-energy ball milling of raw materials of a phosphate compound and corn stalk cores, and then carbon thermal reduction to obtain a biomass carbon composite sodium ion battery positive electrode material. The cellulose and hemicellulose rich in the corn stalk cores are used to realize in-situ coating of carbon, and a fluorovanadium sodium phosphate positive electrode material with a three-dimensional conductive network structure is constructed. The first discharge specific capacity of the biomass carbon composite fluorovanadium sodium phosphate material is 135.60 mAh / g at a 1 C rate, and the discharge specific capacities are 91.25 mAh / g and 76.01 mAh / g at 20 C and 30 C rates respectively. The biomass carbon composite fluorovanadium sodium phosphate material can be used in the field of sodium ion batteries.

Description

Technical Field

[0001] This invention belongs to the field of electrochemical energy storage technology, specifically relating to the preparation method and application of biomass carbon composite sodium cathode material. Background Technology

[0002] Sodium-ion batteries (SIBs) are considered a highly promising large-scale energy storage technology due to the abundance and low cost of sodium and their similar operating mechanism to lithium-ion batteries, and have received significant attention from academia and industry in recent years. Cathode materials, as key components determining the electrochemical performance, cost, and cycle life of SIBs, are crucial to research and development. Ideal SIB cathode materials should possess high specific capacity, high operating voltage, excellent cycle stability, and rate performance, while their synthesis process must be low-cost, environmentally friendly, and scalable.

[0003] Among numerous candidate materials, phosphate compounds exhibit promising application prospects due to their high operating voltage plateau, high specific energy density and volumetric energy density, and stable NASICON-type crystal structure. However, phosphate materials inherently suffer from low intrinsic electronic conductivity, slow sodium ion diffusion kinetics, low actual specific capacity, and insufficient rate performance, severely limiting their practical application in high-power scenarios. To overcome these shortcomings, a strategy of combining phosphates with carbon materials can effectively enhance electron transport capabilities, suppress particle aggregation, and alleviate side reactions at the electrode / electrolyte interface, thereby significantly improving their electrochemical performance, especially under high-rate charge-discharge conditions.

[0004] Currently, carbon composite technologies used in cathode materials mainly include in-situ carbonization and ex-situ coating. In-situ carbonization typically involves introducing an organic carbon source during material synthesis, followed by heat treatment to achieve in-situ growth of the carbon layer. Ex-situ coating requires mixing pre-synthesized active materials with a carbon source (such as glucose or phenolic resin) to form a suspension, and then forming a carbon coating layer under an inert atmosphere using methods such as spray pyrolysis, high-temperature pyrolysis, or chemical vapor deposition. However, these processes generally suffer from problems such as complex procedures, high energy consumption, reliance on expensive or non-renewable carbon sources, and harsh reaction conditions. Furthermore, they are often multi-step processes, which not only increase material costs but also enhance the complexity of electrode fabrication, hindering low-cost and green manufacturing of battery systems. Therefore, exploring the use of renewable and low-cost biomass resources as carbon sources to achieve in-situ carbon coating and heteroatom doping of phosphate materials through simple, efficient, and energy-saving processes to construct a three-dimensional conductive network structure not only helps improve the overall electrochemical performance of the materials but also aligns with the development direction of resource recycling and green manufacturing, possessing significant research value and application prospects. Summary of the Invention

[0005] This invention aims to address the technical challenges of complex and costly carbon composite modification processes for existing phosphate-based sodium-ion battery cathode materials, thereby providing a method for preparing and applying biomass-carbon composite sodium-ion battery cathode materials. Using waste corn stalks as a carbon source, this invention employs a simple in-situ synthesis process to simultaneously construct carbon-coated phosphate-based sodium-ion battery cathode materials and a three-dimensional conductive network. This fundamentally enhances the electron transport capability and ion diffusion rate of the cathode material, meeting the demands for high-rate performance and long cycle life. Specifically, the method of this invention does not rely on complex post-processing or external doping methods. Instead, it utilizes high-energy ball milling and one-step carbothermal reduction to directly achieve in-situ growth and composite of the carbon layer using corn stalk precursors during the material synthesis process. The carbon material derived from waste corn stalks not only provides a good conductive network, but its natural porous structure also offers abundant channels for ion transport, thus synergistically improving the overall electrochemical performance of the material.

[0006] The preparation method of the biomass carbon composite sodium-ion battery cathode material of the present invention is carried out according to the following steps:

[0007] 1. The waste corn stalks are washed with water, acid and alkali respectively, and then washed until neutral to remove impurities. Then they are dried and crushed to obtain corn stalk powder.

[0008] 2. Weigh the raw materials according to the stoichiometric ratio of phosphate compounds, and then weigh corn stalk powder according to 6% to 10% of the total mass of the phosphate compound raw materials; after mixing evenly, a mixed powder is obtained.

[0009] 3. Load the mixed powder into a ball mill jar and ball mill for 5 to 8 hours at a ball-to-powder mass ratio of (20~30):1 and a rotation speed of 1200~1500 rpm to achieve uniform mixing of each component at the molecular / atomic level and obtain the precursor.

[0010] IV. The precursor is placed in a tube furnace and calcined at 400-600℃ for 4-8 hours under an argon atmosphere, followed by natural cooling to obtain a biomass-carbon composite sodium-ion battery cathode material. During this process, corn stalk cores undergo in-situ carbonization to form a carbon layer coating the surface of phosphate compound particles, and the crystalline phase of the phosphate compound is synthesized simultaneously, forming a phosphate compound and biomass-carbon composite material. This invention utilizes a simple "ball milling-calcination" process to simultaneously achieve carbon coating and the construction of a three-dimensional porous conductive network using waste biomass. This composite structure not only significantly improves the material's electronic conductivity and accelerates charge transport, but its rich porous structure also provides channels for the rapid diffusion of sodium ions and exposes more electrochemical active sites, thereby synergistically improving the material's specific capacity, rate performance, and cycle stability.

[0011] Furthermore, the acid washing described in step one involves soaking the waste corn stalk cobs in a 2 mol / L hydrochloric acid solution for 10-12 hours.

[0012] Furthermore, the alkaline washing described in step one involves soaking the waste corn stalk cobs in a 5% ammonia solution for 10-12 hours.

[0013] Furthermore, the phosphate compound mentioned in step two is sodium vanadium fluorophosphate, sodium vanadium phosphate, or sodium iron pyrophosphate.

[0014] Furthermore, when the phosphate compound mentioned in step two is sodium vanadium fluorophosphate, the raw materials include a sodium source, a vanadium source, a phosphorus source, and a fluorine source; wherein the sodium source is sodium carbonate; the vanadium source is vanadium pentoxide; the phosphorus source is ammonium dihydrogen phosphate; and the fluorine source is ammonium fluoride.

[0015] The application of the biomass carbon composite sodium-ion battery cathode material prepared by the above method is to prepare the biomass carbon composite sodium-ion battery cathode material into an electrode sheet for use as the cathode of a sodium-ion battery.

[0016] Furthermore, when preparing a sodium-ion battery cathode using biomass carbon composite sodium-ion battery cathode material, the biomass carbon composite sodium-ion battery cathode material, conductive agent Super P and binder polyvinylidene fluoride (PVDF) are mixed evenly in a mass ratio of 8:1:1 to form a slurry, which is then coated onto a current collector and dried to obtain a sodium-ion battery cathode sheet.

[0017] This invention utilizes a simple "ball milling-calcination" process to simultaneously achieve carbon coating and the construction of a three-dimensional porous conductive network using waste corn stalks. Corn stalks are rich in cellulose, hemicellulose, and natural nitrogen, making them an ideal precursor for preparing functional carbon materials. Through high-energy ball milling and a simple carbothermal reduction method, in-situ growth of the carbon layer can be achieved, utilizing its excellent conductivity to improve the material's electron transport capability. The corn stalks possess a well-developed porous structure and a large specific surface area, providing abundant reaction interfaces and ion transport channels for the active material sodium vanadium fluorophosphate after carbonization, facilitating the rapid diffusion of sodium ions. Simultaneously, the porous structure of the corn stalk carbon can construct a highly efficient three-dimensional conductive network. This composite structure not only significantly improves the material's electronic conductivity and accelerates charge transport, but its rich porous structure also provides channels for the rapid diffusion of sodium ions and exposes more electrochemical active sites, thereby synergistically improving the material's specific capacity, rate performance, and cycle stability. The process for preparing electrodes using the composite cathode material of this invention is the same as the conventional electrode preparation process. Specifically, the composite cathode material, conductive agent, and binder are mixed uniformly to form a slurry, which is then coated onto a current collector and dried to obtain the electrode sheet. Because the composite material of this invention possesses an excellent conductive network, the amount of conductive agent added can be reduced.

[0018] The advantages of the biomass carbon composite sodium-ion battery cathode material of the present invention compared with ordinary sodium vanadium fluorophosphate are as follows:

[0019] (1) This invention uses corn stalk cores as a carbon source and employs a one-step synthesis technique involving pre-purification and ball milling-calcination to successfully prepare a sodium-ion battery cathode material composed of biomass carbon and sodium vanadium fluorophosphate. This method is simple, energy-efficient, and operates under mild conditions, significantly improving processability and industrial mass production potential. Traditional sodium vanadium fluorophosphate carbon composite processes generally suffer from problems such as cumbersome procedures, high energy consumption, numerous steps, and harsh reaction conditions. They typically require complex processes such as multi-step heat treatment, suspension preparation, spray drying, or chemical vapor deposition, which are not conducive to large-scale production. This invention adopts an integrated in-situ synthesis strategy of ball milling-calcination, eliminating the need for complex intermediate processing and high-energy-consuming equipment. It can achieve uniform composite of sodium vanadium fluorophosphate precursor and corn stalk core carbon source, in-situ carbothermal reduction, and simultaneous carbon layer formation at relatively low temperatures. The process is short, simple to operate, has good reproducibility, and lower energy consumption. This method not only significantly reduces production difficulty and manufacturing costs, but also ensures batch stability and structural uniformity of materials, solving the pain point that traditional carbon coating processes are difficult to scale up industrially. It is more suitable for the core needs of the large-scale energy storage industry of sodium-ion batteries for low-cost, high-efficiency, green and mass-producible cathode materials, and its industrial application prospects are far superior to ordinary sodium vanadium fluorophosphate materials.

[0020] (2) Benefiting from the high conductivity of corn stalk core carbon material, the bottleneck of intrinsic conductivity and ion transport of sodium vanadium fluorophosphate is fundamentally solved. Ordinary sodium vanadium fluorophosphate materials have extremely low electronic conductivity and slow sodium ion diffusion kinetics, resulting in inherent defects such as easy structural decay, poor rate performance, and insufficient long-cycle stability during charge and discharge, making it difficult to meet the requirements of high-power and long-life energy storage applications. This invention uses waste corn stalk cores as biomass carbon source and forms a continuous, uniform, and highly conductive carbon coating layer on the surface of sodium vanadium fluorophosphate particles through in-situ carbothermic reduction reaction. At the same time, it retains the unique multi-level porous structure and three-dimensional conductive network of corn stalk cores after carbonization, which can significantly shorten the electron transport path, accelerate the sodium ion diffusion rate, and significantly improve the rate performance and cycle stability of the material. The sodium-ion battery cathode material of biomass carbon composite sodium vanadium fluorophosphate prepared by this invention has satisfactory rate performance. At room temperature and 1 C rate, the sodium-ion half-cell capacity assembled by the biomass carbon composite sodium vanadium fluorophosphate cathode material prepared by this invention is as high as 135.60 mAh / g. In particular, it maintains a discharge specific capacity of approximately 76.01 mAh / g at a high rate of 30 C, which is significantly better than uncoated sodium vanadium fluorophosphate materials (typically below 40 mAh / g). After 1000 cycles at 10 C, the capacity retention exceeds 80%, demonstrating excellent long-term cycling stability.

[0021] (3) Achieving low-cost raw material utilization and resource utilization of agricultural waste, combining economic and environmental advantages. Traditional carbon-coated sodium vanadium fluorophosphate often uses petrochemical-based or chemical carbon sources such as glucose, sucrose, and phenolic resin, which are not only costly and non-renewable, but also require the addition of dopants. This invention uses waste corn stalks as the sole carbon source, which is widely available, inexpensive, and easy to obtain, truly realizing "turning waste into treasure" and significantly reducing the production cost of cathode materials from the source. At the same time, direct incineration of corn stalks easily causes air pollution and resource waste. This invention utilizes them in a high-value manner, which reduces environmental pollution and conforms to the dual carbon goals and resource recycling policy. Corn stalks are rich in natural polymers such as cellulose and hemicellulose. The porous biomass carbon formed after carbonization has high conductivity, high specific surface area, and self-doping characteristics. It can achieve efficient carbon coating without complex modification, and its comprehensive benefits are significantly better than ordinary sodium vanadium fluorophosphate materials using traditional carbon sources.

[0022] The method of this invention is simple and can complete the preparation of composite cathodes through conventional mixing, paste making and coating steps. It is highly compatible with existing sodium-ion battery electrode preparation systems, does not rely on special equipment or exogenous additives, has good versatility and operational stability, is easy to scale up and apply directly on existing battery production lines, and has high industrial transformation potential. Attached Figure Description

[0023] Figure 1 X-ray diffraction patterns of sodium-ion battery cathode materials prepared in Example 1, Comparative Example 1, Comparative Example 2, and Comparative Example 3;

[0024] Figure 2 Scanning electron microscope (SEM) images of the sodium-ion battery cathode materials prepared in Example 1 and Comparative Example 1;

[0025] Figure 3 Transmission electron microscope (TEM) image of the biomass carbon composite sodium vanadium fluorophosphate cathode material prepared in Example 1;

[0026] Figure 4 Raman spectra of sodium-ion battery cathode materials prepared in Example 1, Comparative Examples 1 and 3;

[0027] Figure 5 The rate performance diagrams are for the sodium-ion battery cathode materials prepared in Example 1, Comparative Example 1, Comparative Example 2, and Comparative Example 3.

[0028] Figure 6 The cycling performance of the biomass carbon composite sodium vanadium fluorophosphate cathode material prepared in Example 1 and the sodium vanadium fluorophosphate cathode material prepared in Comparative Example 1 at a 1 C rate is shown in the graph.

[0029] Figure 7The graph shows the cycle performance of the biomass carbon composite sodium vanadium fluorophosphate cathode material in Example 1 at -20°C.

[0030] Figure 8 The graph shows the cycle performance of the biomass carbon composite sodium-ion battery cathode material prepared in Example 1 at a high temperature of 60 °C.

[0031] Figure 9 The graph shows the full-cell performance of the biomass carbon composite sodium-ion battery cathode material in Example 1 at 25°C. Detailed Implementation

[0032] The beneficial effects of the present invention will be verified using the following examples.

[0033] Example 1: The preparation method of the biomass carbon composite sodium-ion battery cathode material in this example is carried out according to the following steps:

[0034] 1. Wash the waste corn stalks several times with deionized water, then dry them in a 60℃ drying oven for 12 hours; then soak them in a 2 mol / L hydrochloric acid solution and a 5% ammonia solution for 12 hours each, filter them, wash them with deionized water until pH=7, and then dry them at 60℃ for 12 hours. After crushing, corn stalk powder is obtained.

[0035] 2. Weigh out 0.397 g of anhydrous sodium carbonate, 0.575 g of ammonium dihydrogen phosphate, 0.453 g of vanadium pentoxide, and 0.310 g of ammonium fluoride according to the stoichiometric ratio of sodium vanadium fluorophosphate (Na3V2(PO4)2F3). Then weigh out 0.174 g of corn stalk powder, wherein the mass of the corn stalk powder is 10% of the total mass of the sodium vanadium fluorophosphate precursor. After mixing evenly, a mixed powder is obtained.

[0036] 3. The mixed powder is loaded into a ball mill jar and ball milled for 5 hours at a ball-to-powder mass ratio of 30:1 and a rotation speed of 1200 rpm. The mixture is paused for 1 minute every hour of ball milling to ensure that the components are uniformly mixed at the molecular / atomic level to obtain the precursor.

[0037] Fourth, the precursor is placed in a tube furnace and calcined at 600℃ for 8 hours under an argon atmosphere, followed by natural cooling to obtain a biomass carbon composite sodium-ion battery cathode material. During this process, corn stalk cores undergo in-situ carbonization to form a carbon layer coating the surface of phosphate compound particles, and the synthesis of phosphate compound crystalline phases is simultaneously completed, forming a phosphate compound and biomass carbon composite material.

[0038] Comparative Example 1: This comparative example differs from Example 1 in that the amount of corn stalk powder added in step two is 0. Other steps and parameters are the same as in Example 1, and sodium vanadium fluorophosphate, a cathode material for sodium-ion batteries, is obtained.

[0039] Comparative Example 2: This comparative example differs from Example 1 in that the amount of corn stalk core powder added in step two is 0.104 grams. The other steps and parameters are the same as in Example 1, resulting in a biomass carbon composite sodium-ion battery cathode material.

[0040] Comparative Example 3: This comparative example differs from Example 1 in that in step two, the corn stalk core powder is replaced with the same mass of anhydrous citric acid. The other steps and parameters are the same as in Example 1, resulting in a carbon composite sodium vanadium fluorophosphate cathode material for sodium-ion batteries.

[0041] Electrode sheets and sodium-ion half-cells were prepared using the sodium-ion battery cathode materials prepared in Example 1, Comparative Examples 1, 2, and 3. The specific steps are as follows:

[0042] The sodium-ion battery cathode materials prepared in Examples 1, 1, 2, and 3 were used as cathode materials. They were thoroughly mixed at a mass ratio of 8:1:1 (cathode material, conductive carbon black (Super P), and PVDF binder), and an appropriate amount of NMP solvent was added. The mixture was magnetically stirred for 4 hours to form a slurry, which was then uniformly coated onto a current collector aluminum foil and dried in a vacuum drying oven at 120 °C for 12 hours. The dried cathode sheet was then stamped into a 14 mm diameter disc and used as the cathode sheet. The cathode sheet was then assembled with a sodium metal anode, a glass fiber separator, and 1 mol / L NaClO4 / EC:PC = 1:1 + 5% FEC to form a CR2025 type sodium-ion half-cell.

[0043] Figure 1 X-ray diffraction patterns of the sodium-ion battery cathode materials prepared in Examples 1, 1, 2, and 3. Figure 1 It can be seen that the positions of the main diffraction peaks of all samples are highly consistent with the JCPDS No. 89-8485 standard card, indicating that sodium vanadium fluorophosphate crystals were successfully synthesized from all four materials, and the main phase structure did not change due to the different types or amounts of carbon sources. The diffraction peak of Comparative Example 1 is the strongest and sharpest, indicating that it has the highest crystallinity, but the lack of carbon coating may lead to poor conductivity. The peak intensities of Example 1 and Comparative Example 2 are slightly lower and the peak shapes are slightly wider, indicating that the introduction of carbon sources may have inhibited the excessive growth of grains to some extent, reducing the crystal size. The diffraction peak intensity of Comparative Example 3 is between that of Comparative Example 2 and Example 1, indicating that the effect of citric acid as a carbon source on the crystallization process is different from that of biomass carbon sources. None of the samples showed the typical amorphous carbon (002) broad peak, indicating that the carbon content was low.

[0044] Figure 2 The images show scanning electron microscope (SEM) images of the sodium-ion battery composite cathode materials prepared in Example 1 and Comparative Example 1; where a is the biomass carbon composite sodium-ion battery cathode material of Example 1, and b is the sodium vanadium fluorophosphate cathode material of Comparative Example 1. Figure 2 As can be observed, the corn stalk core carbon material exhibits an overall mixture of irregular particles and lamellar structures, with some areas showing obvious needle-like or rod-like crystals. These needle-like structures are incompletely converted sodium vanadium phosphate or byproducts, with a size of approximately 500 nanometers. Furthermore, the particle surface is rough, with numerous pores and uneven structures, indicating a high specific surface area and abundant micropores, which are beneficial for electrolyte penetration and ion transport. The needle-like structures are interwoven with surrounding blocky or lamellar materials, with some needles embedded in the carbon matrix, suggesting that the carbon material may have played a role in structural support and confined growth during high-temperature processing. No obvious agglomeration or dense bulk structures are observed in the overall structure, indicating good material dispersion; the carbon coating layer may have inhibited excessive grain growth and agglomeration to some extent. Conversely, sodium vanadium fluorophosphate without biomass carbon (… Figure 2 Sample b) exhibits a dense, blocky "stone-like" structure with large particles (1-3 micrometers) that are severely aggregated, with a smooth surface and almost no pores. This difference stems from the dual role of biomass carbon in the preparation process: on the one hand, the carbon framework acts as a physical barrier, inhibiting excessive grain growth and aggregation, thus maintaining the material at the nanoscale; on the other hand, the natural porous structure of corn stalk carbon or the gases generated during pyrolysis provide sites for NVPF nucleation, forming an interconnected porous network. Simultaneously, the carbon itself also constructs a conductive network, enhancing the material's electrochemical performance. In contrast, the undoped sample, lacking constraint during high-temperature calcination, experiences rapid grain growth and densification, which is detrimental to electrolyte penetration and electron transport.

[0045] Figure 3 Transmission electron microscopy (TEM) image of the biomass-carbon composite sodium-ion battery cathode material prepared in Example 1. Figure 3 Images a and b show that the material consists of numerous irregular particles and needle-like structures. The needle-like structures are approximately tens to hundreds of nanometers in size, consistent with previous SEM observations, and represent the morphology of sodium vanadium fluorophosphate crystals growing along a specific crystal orientation. Figure 3 The crystal lattice fringes are clearly visible in the c-shape, with the (220) crystal plane marked in red. The interplanar spacing d = 0.32 nm, consistent with the standard interplanar spacing of sodium vanadium fluorophosphate, confirms the crystal structure of the material. The high coexistence of Na, V, P, and F elements in the needle-like structure confirms that it is a sodium vanadium fluorophosphate phase; the widespread distribution of carbon indicates that the carbon coating layer covers the entire material. This material successfully achieves a sodium vanadium fluorophosphate carbon-coated structure using corn stalk cores as the carbon source. The crystal structure is complete, the elemental distribution is uniform, and the carbon layer effectively coats the surface of the active material.

[0046] Figure 4 Raman spectra of the sodium-ion battery cathode materials prepared in Example 1, Comparative Example 1, and Comparative Example 3. From... Figure 4 It can be seen from the data that Comparative Example 1 and Comparative Example 3 are both at 900, 944, and 1045 cm.−1 Three peaks appeared at 1357 and 1578 cm⁻¹, corresponding to the characteristic peaks of sodium vanadium fluorophosphate. Because the biomass carbon shielded the signal, the characteristic peaks of sodium vanadium fluorophosphate did not appear in the Raman spectrum of Example 1, indicating that the biomass carbon effectively coated the sodium vanadium fluorophosphate. Furthermore, Example 1 and Comparative Example 3 showed peaks at 1357 and 1578 cm⁻¹. −1 Two peaks also appeared at the location, which can be attributed to the carbon A in the composite material. 1g Vibration modes (D-band) and E 2g Vibration mode (G-band). Integral intensity ratio (I) D / I G () indicates the degree of order in graphite within carbon. Example 1 and Comparative Example 3, I D / I G With values ​​of 0.91 and 1.67 respectively, it can be clearly observed that Example 1 has a high degree of graphitization, which provides a structural basis for improving the conductivity and electrochemical performance of the material.

[0047] Figure 5 The graph shows the rate performance of sodium-ion half-cells assembled from the sodium-ion battery cathode materials prepared in Examples 1, 1, 2, and 3 at 25 °C. Figure 5 It can be seen that in the range of 2.5~4.5 V (vs. Na) + Within the voltage range of / Na), Example 1 exhibited the best rate performance and cycling stability, maintaining a high specific capacity across a wide rate range from 0.1 C to 30 C. Comparative Example 2 showed a slightly lower capacity, indicating that the amount of corn stalk added affected performance, but overall it was still superior to other carbon sources. Comparative Examples 3 and 1 performed poorly, with rate performance failing to meet practical requirements. Especially at a high rate of 30 C, Example 1 still achieved a capacity as high as 76.01 mA hg. -1 It is much higher than that of Comparative Example 1 (6.35 mA hg). -1 ) and Comparative Example 3 (7.18 mA hg) -1 Furthermore, Example 1 exhibits extremely strong recovery capability after high-rate cycling, indicating that its carbon coating layer has good conductivity and structural stability, effectively improving ion transport and electronic conduction capabilities. The fundamental reason for the excellent rate performance of Example 1 lies in the synergistic effect of the highly efficient conductive network formed by corn stalk core as a biomass carbon source and the nanostructure: the carbon layer formed after carbonization not only uniformly coats the surface of sodium vanadium fluorophosphate crystals, improving electronic conductivity, but also optimizes the structural stability of the material due to the natural porous characteristics of corn stalk core; at the same time, the nanoscale particle size significantly shortens the ion diffusion path, enabling rapid transport of electrons and ions under high-rate charge and discharge, thus maintaining high specific capacity while exhibiting excellent rate adaptability and cycle stability, making it suitable for high-performance sodium-ion battery cathode materials.

[0048] Figure 6 The graphs show the cycling performance of sodium vanadium fluorophosphate (CNF) cathode materials prepared in Example 1 and Comparative Example 1 at a 10 C rate. A significant difference in performance is observed between the two. Within the voltage range of 2.5–4.5 V (vs. Na+ / Na), at a 10 C rate, the cathode material of Example 1 exhibits an initial discharge specific capacity of 116.55 mAh / g, and after 1000 cycles, a discharge specific capacity of 97.31 mAh / g, with a cycle retention rate of 83.5%, demonstrating excellent cycling performance. In contrast, the sodium ion battery cathode material prepared in Comparative Example 1 exhibits an initial discharge specific capacity of 56.53 mAh / g at a 10 C rate, and after 1000 cycles, a discharge specific capacity of 17.85 mAh / g, with a cycle retention rate of 31.5%, indicating poor cycling performance. Comparative Example 1, lacking a carbon source, exhibited poor material conductivity and was prone to structural collapse during cycling, resulting in rapid capacity decay. In contrast, Example 1, using corn stalk cores as a carbon source, formed a uniform carbon coating layer that significantly improved electronic conductivity, effectively buffered volume expansion, and stabilized the crystal structure, thereby maintaining high capacity and excellent capacity retention during long-term cycling.

[0049] Figure 7 The graph shows the cycling performance of the biomass carbon composite sodium-ion battery cathode material prepared in Example 1 at a low temperature of -20 °C. It can be seen that, at -20 °C, the sodium vanadium fluorophosphate prepared in Example 1 can provide an initial discharge specific capacity of 111.56 mAh / g at a 1 C rate, and after 500 cycles, its discharge specific capacity is 101.07 mAh / g, with a capacity retention rate as high as 90.6%.

[0050] Figure 8 The graph shows the cycling performance of the biomass-carbon composite sodium-ion battery cathode material prepared in Example 1 at a high temperature of 60 °C. It can be seen that at 60 °C, the electrode material prepared in Example 1 provides an initial discharge specific capacity of 132.02 mAh / g at a 10 C rate, and after 200 cycles, its discharge specific capacity is 110.51 mAh / g, with a cycle retention rate of 83.6%. Figure 7 and Figure 8 It can be seen that the biomass carbon composite sodium-ion battery cathode material prepared in Example 1 has good wide temperature range performance, is more adaptable to extreme environments, and has good application prospects.

[0051] To verify the practical application value of the prepared biomass-carbon composite sodium vanadium fluorophosphate electrode material, it was coupled with a commercial hard carbon anode to assemble a full cell. The galvanostatic charge-discharge curve of the full cell is shown below. Figure 9As shown, the initial discharge capacity of the biomass carbon composite sodium-ion battery cathode material (|| hard carbon full cell) at 1C rate reaches as high as 124.94 mAh / g. Furthermore, the biomass carbon composite sodium-ion battery cathode material (|| hard carbon full cell) also exhibits good cycle stability, maintaining 80% capacity retention after 1000 cycles at 1C rate, thanks to the excellent rate performance of the biomass carbon composite sodium vanadium fluorophosphate. In summary, this full cell fully utilizes the high energy density characteristics of the biomass carbon composite sodium vanadium fluorophosphate, demonstrating excellent electrochemical performance. Therefore, the biomass carbon-optimized sodium vanadium fluorophosphate material prepared using corn stalk cores as a carbon source has significant value in enhancing the commercial application prospects of sodium-ion batteries.

[0052] This invention addresses the shortcomings in rate capability and cycle performance of phosphate-based cathode materials for sodium-ion batteries due to their low intrinsic electronic conductivity and slow sodium-ion diffusion kinetics. It innovatively utilizes corn stalk cores, an agricultural waste, as a biomass carbon source, and prepares composite materials in situ using a simple ball milling-calcination technique. Corn stalk cores are rich in cellulose, hemicellulose, and natural nitrogen. The porous carbon framework formed after carbonization not only constructs a highly efficient three-dimensional conductive network, significantly improving electron transport capacity and ion diffusion rate, but also effectively buffers volume expansion and stabilizes the crystal structure through in-situ coating, thereby synergistically optimizing the material's specific capacity, rate performance, and long-cycle stability. This process combines low cost, environmental friendliness, and scalability, achieving high-value utilization of agricultural waste and providing a new pathway for the development of high-performance, green sodium-ion battery cathode materials.

Claims

1. A method for preparing a biomass carbon composite sodium-ion battery cathode material, characterized in that, This method is performed in the following steps:

1. The waste corn stalks are washed with water, acid and alkali respectively, and then washed until neutral to remove impurities. Then they are dried and crushed to obtain corn stalk powder.

2. Weigh the raw materials according to the stoichiometric ratio of phosphate compounds, and then weigh corn stalk powder according to 6% to 10% of the total mass of the phosphate compound raw materials; after mixing evenly, a mixed powder is obtained.

3. Load the mixed powder into a ball mill jar and ball mill for 5 to 8 hours at a ball-to-powder mass ratio of (20~30):1 and a rotation speed of 1200~1500 rpm to achieve uniform mixing of each component at the molecular / atomic level and obtain the precursor. Fourth, the precursor is placed in a tube furnace and calcined at 400~600℃ for 6~8 hours under argon atmosphere protection, and then naturally cooled to obtain the biomass carbon composite sodium-ion battery cathode material.

2. The method for preparing a biomass carbon composite sodium-ion battery cathode material according to claim 1, characterized in that, The phosphate compound mentioned in step two is sodium vanadium fluorophosphate, sodium vanadium phosphate, or sodium iron pyrophosphate.

3. A method for preparing a biomass carbon composite sodium-ion battery cathode material according to claim 1 or 2, characterized in that, The phosphate compound mentioned in step two is sodium vanadium fluorophosphate, and the raw materials include sodium source, vanadium source, phosphorus source and fluorine source; wherein the sodium source is sodium carbonate; the vanadium source is vanadium pentoxide; the phosphorus source is ammonium dihydrogen phosphate; and the fluorine source is ammonium fluoride.

4. The application of the biomass carbon composite sodium-ion battery cathode material prepared by the method of claim 1, characterized in that, This application involves preparing a sodium-ion battery cathode material composed of biomass carbon into an electrode sheet for use as the cathode of a sodium-ion battery.

5. The application of the biomass carbon composite sodium-ion battery cathode material according to claim 4, characterized in that, The method for preparing sodium-ion battery cathodes using biomass carbon composite sodium-ion battery cathode materials is as follows: a slurry is prepared by uniformly mixing biomass carbon composite sodium-ion battery cathode material, conductive agent Super P and binder polyvinylidene fluoride in a mass ratio of 8:1:1, coating it onto a current collector, and drying it to obtain a sodium-ion battery cathode sheet.

Images